Mandrel for extruding tubing

ABSTRACT

A mandrel and mandrel-dummy block assembly for extrusion of tubing is disclosed wherein the mandrel has a shaft cross sectional area so related to the working section cross sectional area that the mandrel will stay in place without attachment to the dummy block. Such mandrel reduces the problems of mandrel necking down and breakage.

United States Patent [191 Braeuninger MANDREL FOR EXTRUDING TUBING [76]Inventor: Karl F. Braeuninger, 535 Plantmore Dr., Ferguson, Mo. 63135[22] Filed: Mar. 24, 1972 [21] Appl. No.: 237,690

Related US. Application Data [62] Division of Ser. No. 55,531, July 16,1970.

[52] US. Cl. 72/264, 72/370, 72/482 [51] Int. Cl. B21c 23/04 [58] Fieldof Search 72/264, 370, 266, 479,

[56] References Cited UNITED STATES PATENTS 1/1964 Hoffman 72/266 [1113,826,122 [45 July 30,1974

3,526,119 9/1970 Wassen 72/264 Primary ExaminerCharles W. LanhamAssistant Examiner-Robert M. Rogers Attorney, Agent, or Firm-Stephen S.Grace; Robert W. Selby 57 ABSTRACT A mandrel and mandrel-dummy blockassembly for extrusion of tubing is disclosed wherein the mandrel has ashaft cross sectional area so related to the working section crosssectional area that the mandrel will stay in place without attachment tothe dummy block. Such mandrel reduces the problems of mandrel neckingdown and breakage.

6 Claims, 6 Drawing Figures PATENTED O 3. 826. l 22 INVENTOR. {2y 4Kar/F Braeum'nger PAnsmmmown 3.826.122

SHEEI 2 OF 2 Con fa/her Ram Bil/e7 I o 5 Uummy b/OCA Con {Oi/vet Ram71,56 Mano re/ Dummy /ock INVENTOR.

Kar/E Braeun/nyer IQTTORNEY BACKGROUND OF THE INVENTION I-Iollow tubingof metal or plastic materials is commonly made by extruding a solidbillet around a short mandrel held in position in a surrounding femaledie member by a supporting bridge or porthole" element which extends ashort distance into the container of the extrusion press. Suchstructures require that the extruding material must be separated intotwo or more streams" which must be joined together again in the die.Sometimes the resulting weld zones are undesirable.

If truly seamless tubing is desired, it is extruded from hollow billetsaround a mandrel passing through the entire billet and secured to theram or a dummy block ahead of the ram. Such mandrels are commonlysecured to the ram by threaded joints, bolted retainer plates, T-belts,or a hammer head enlargement behind a flange on the ram. The latterprovides a floating mandrel which seeks its own position centered in thefemale die.

It is known that mandrels thus retained on the ram frequently breakbecause the frictional forces in the die exceed the tensile strength ofthe mandrel. This may happen for various reasons. For example, thebillet may be pushed to an excessively small butt left inside thecontainer, the metal temperature may be too low (requiring excessivelyhigh extrusion pressure), the mandrel temperature may be too high(lowering its tensile strength), the mandrel size must be so small thatit has intrinsically low strength, or the mandrel may be subjected tobending stresses as well as tensile stresses due to improper mounting oreccentrically positioned hole in the billet. Naturally, broken mandrelsare an expensive problem because of costly down time on the press, lossof metal scrapped, and the cost of new mandrels.

An object of the present invention is to provide a mandrel andmandrel-dummy block assembly which reduces or overcomes the breakage andother problems presently encountered.

THE INVENTION The present invention relates to a mandrel having aworking section and an enlarged shaft wherein the shaft averagecross-sectional, area (A,) is about equal to where A is the averagecross sectional area of the working section, P is the maximum fluostaticpressure in the container of the extrusion machine, and S is thefrictional stress on the working section during extrusion. The workingsection is that portion of the mandrel which is in contact with thehollow billet as it is formed into tubing through an extrusion die.

Such a mandrel can be held secure in a plain socket or hole on a ram ordummy block by the fluid pressure of the material being extruded actingagainst an enlarged shaft of the mandrel where it enters the socket,i.e., the mandrel need not be attached to the ram or dummy block. Thesize of the enlargement can be calculated such that product of theenlargement of the cross section of the mandrel shaft (A, A,) multiplied by the fluid pressure in the container (P) about equals thefrictional forces (A 8) normally tending to pull the mandrel through thedie but is slightly lower than the tensile strength of the mandrel. Theresult of this proper balance of forces is that during normal extrusionthe mandrel is retained in its socket by the back" pressure on itsshaft. This also permits the mandrel to move forward a short distanceand then back without necking or breakage. However, if unexpectedconditions arise to increase the forces tending to pull the mandrelthrough the die, such forces will be resisted only until the tensilestrength of the mandrel is approached. Then the unexpected forces willpull the mandrel from its socket rather than fracturing it or necking.it down smaller than dimensional tolerances.

The enlargement of the mandrel shaft required for proper retaining inits socket according to my invention is usually small enough that if themandrel is pulled from its socket completely by excessive tensilestresses it will simply pass through the die without damaging the die orcausing sudden destructive extrusion pressures to be built up. Ifcalculations show that the enlargement of cross section must exceed thesize of the opening in the female die member and such a mandrel ispulled from its socket by excessive tensile stresses it will simplylodge against the inner die face to plug it off. This mayv cause anincrease in pressure in the container but this pressure is under ratherconstant control and can be relieved by the press operator before it isharmful to other press equipment, the mandrel or the die.

The applicant has furthermore found that if the socket for the mandrelshaft tapers slightly outwardly from the face bearing against the billetor slightly in back of said face to the face bearing against the ram,the mandrel can have a gyro pendulum motion, i.e., can adjust itself inany direction into the center point of metal flow. Alternatively, theshaft section can be tapered inwardly from front to back to permit suchpendulum motion.

The enlargement of the mandrel shaft can be stepwise but is preferably agradual straight or curved taper. The retaining pressure depends onlyupon the difference in area between the working section and its shaft. Acurved taper is best because it avoids stress concentrations associatedwith abrupt changes in diameter.

In order to facilitate removal of the mandrel from the butt end of thebillet after tubing has been extruded, the mandrel preferably has ashort, raised shoulder portion of the working section next to the shaft.The shoulder cross sectional area is intermediate between the workingsection area and the shaft area.

FIG. 1 is a drawing of one embodiment of the mandrel of the presentinvention.

FIG. 2 is a drawing of a back face of block which can be used incombination with the mandrel of the present invention.

FIG. 3 is the cross section of the dummy block of FIG. 2 taken alongline 3-3.

FIG. 4 is a drawing of another embodiment of the mandrel of the presentinvention.

FIG. 5 is a schematic diagram of the extrusion equipment including anembodiment of the mandrel of the present invention and billet at thestart of a push.

FIG. 6 is a schematic diagram of the extrusion equipment including anembodiment of the mandrel of the present invention and billet at the endof a push.

a loose dummy Referring to FIG. 1, the mandrel comprises a workingsection, denoted generally as 1, and a shaft, 2. The working section hasa nose portion 3 and raised shoulder 4. The working section is normallytapered outward slightly from nose to tail. An actual mandrel of thisdesign made for extruding magnesium alloy tubing had the followingdimensions. 0.785 inch diameter at the nose of the working section,0.793 inch diameter at just below the shoulder section, 0.805 inchshoulder diameter, 1.250 inch shaft diameter, 3 inch long shaft, l2 inchlong working section.

Such a mandrel can be used in an extrusion machine in combination withan attached dummy block (not shown) or loose dummy block such as shownin FIGS. 2-3. The dummy block 5 has a socket concentric with the centerof the dummy block cross section. Such socket is tapered outwardly fromslightly in back of the face bearing against the billet 7 (this givesless wear on the socket surface than if the taper were complete) to theface bearing against the ram 8. Such taper permits the end of the noseof the mandrel to move in any direction perpendicular to the directionof the extrusion (gyro-pendulum motion) and prevents flexing of themandrel in case of off center billets or erratic metal flow through thedie. This avoids the combined effect of friction and bending stresses.

Another embodiment of the mandrel of the present invention is shan inFIG. 4. In tis embodiment the shaft 2 is tapered from front to rear andcontains a threaded hole 9.

The mandrel of the present invention can be utilized in the followingmanner to extrude tubes of metal, plastic or similar materials. Theextrusion is carried out on a conventional extrusion machine having anextrusion container, tubing die and movable ram. Where a loose dummyblock is used, the hollow billet and dummy block are loaded into theextrusion container. The working section of the mandrel, normallylubricated and heated, is inserted through the socket of the dummy blockinto the billet. The ram is advanced and the tube extruded. The relativeposition of the extrusion equipment and billet at the start of the pushis shown in FIG. 5. The mandrel is not attached to the ram or dummyblock but merely seated in the dummy block socket. As the ram is movedforward the mandrel, having the prescribed shaft to working section arearelationship, is held in position by the fluid pressure in the containeracting on the shaft. FIG. 6 shows the position of the extrusionequipment and billet at the end of the push. The billet is reduced to asmall butt end. The shoulder of te working section is wedged in thebutt. The die and ram are pulled away. The mandrel is removed by e.g.,screwing a ring into the threaded hole 9 in the mandrel shaft (FIG. 4)and attaching a chain to the ring and the movable extrusion frame whichmoves the ram. The frame is then moved back pulling the mandrel out ofthe butt through the dummy block.

Where an attached dummy block is used, the procedure is similar. Thehollow billet is loaded into the extrusion container and the nose of themandrel inserted into the billet. The ram having the dummy blockattached thereto is moved into place such that the mandrel shaft ispositioned in the dummy block socket. The ram is then further advancedto extrude the. tube.

The mandrel of the present invention is particularly suited forextruding small inside diameter metal tubing which cannot practicallybeextruded by conventional techniques due to the-tendency of the mandrelto neck down and break. Especially difficult extrusions of tubes witheither extremely thin-or extremely heavy walls are successfully madeusing the present mandrel.

The basic equation for the area of the shaft which will permit themandrel to begin to move forward'out of the dummy block due to thefrictional force of the material flowing forward over the mandrel iswhere A area of mandrel shaft A average cross sectional area of workingsection of mandrel P maximum fluostatic pressure in the container,dependent on the press capacity and container diameter.

S stress on the working section developed by friction.

The shaft diameter may be readily calculated from A in Equation (1). Toolarge a shaft diameter will restrict the mandrel movement and permit thenecking'typical of the conventional die mandrel process, whereas toosmall a shaft diameter will permit the mandrel to be sucked through thedie at the end of the push. A shaft diameter slightly smaller thanoptimum is to be preferred to one too large, since the undamaged mandrelcan be retrieved from the inside of the die if it is sucked through thedie.

S=F/A where F =force exerted on mandrel by friction. At any giveninstant during extrusion F P 2 1TD1 where P, fluostatic pressure in thecontainer at the time in question D diameter of working section ofmandrel L billet length at the time in question p. coefficient offriction between the extruded material and mandrel. Also at any instantduring extrusion P2 ug. LID2) where P, pressure on the die D containerdiameter e naperian logarithm base. Therefore, letting x 4p.L/D

F P 6 LID2)7TD1LI.L P,,(e")'rrD, X D u/4p.

= P,1rD,D (e)x/4 In order to determine F throughout the push, it isnecessary to set up the following differential equation.

Solving the differential equation between limits where x, 4p.L,/D,, Lbeing the length of the billet at the end of the push (butt).

x 4uL /D L being the length of the billet at the start of the push.Integrating and rearranging terms,

F: POWDIIL w: L2/D2 up L1ID2).]

The generally accepted formula for the pressure at the die is P BY log Rwhere Y tensile yield strength of the material being extruded (in thepresent example, alumina), at the temperature of extrusion; assumed tobe percent of the room temperature yield strength for thesecalculations.

R extrusion reduction ratio. B a constant dependent on the extrusion dieparameters.

where K a constant established as 4 for all light alloys.

8, bearing length of the die for the tube being extruded. (Themultiplier 5.5 takes into consideration the bearing effect of themandrel.)

B bearing length for a round rod having the same cross sectional area asthe tube being extruded.

C, O.D. circumference of tube being extruded.

C [.D. circumference of tube being extruded.

C circumference of round rod having the same cross sectional area as thetube being extruded.

In order to calculate the actual diameter of a mandrel shaft, it isnecessary, of course, to utilize the above equations in reverse order.Considering the case of aluminum tubing having 3.812 inches O.D. X 1.547inches 1.D., where B 0.5 inch, B 1.0 inch, C 11.98 inches, C 4.86 inchesand C 10.93 inches.

B =4 X 5.5 X 0.5(ll.98 4.86)/l.0 X 10.93 16.93

For a tube area of 9.53 sq. in. and a 16 inches diameter container areaof 201 sq. in. (billet 1.D. ignored), the reduction ratio would be 21.1.Assuming Y 0.15 X 3500 psi 525 psi.

P 16.93 X 525 Xlog 21.l 27,100 psi For a 25 inches long billet and 4inches butt, assuming The value of 0.04 for p. assumes good lubrication.1f the lubrication on the mandrel were to break down, t could easilyincrease to a value of 0.10, in hich case F would be 424,000 pounds andS would be 225,000 psi. However the working temperature of the mandrelcould easily exceed 1,000, at which temperature the yield strength ofH11 or H13 tool steel is only about 120,000 psi. Therefore, to avoidbreaking the mandrel, a maximum value of 100,000 psi should be assumedfor S. The fluostatic pressure P in the 16 inches container on the5,500T press could reach 5,500 X 2,000/201 55,000 psi, so

Shaft diameter V 4A V 6.76 2.60 inches To add further safety and insurethat the mandrel would move forward rather than neck down, the shaftdiameter was made 2.500 inches.

What is claimed is:

1. A method of extruding tubes from an extrusion press having a movableram which comprises 7 a. providing a mandrel which has a working sectionhaving a nose and a tail, and a shaft abutting the tail of the workingsection wherein the shaft cross sectional area is about equal to A S/P Awhere A is the average cross sectional area of the working section,

I, is the maximum fluostatic pressure in the container of the extrusionmachine, and

S is the frictional stress on the working section during extrusion b.positioning the nose of the mandrel-working section in hole of a hollowbillet and the shaft in the socket of a dummy block 0. extruding thebillet through a die to form a tube.

. 2. The method of claim 1 wherein the extruded tubes are metal.

3. The method of claim 2 wherein the metal is selected from the groupconsisting of aluminum alloys and magnesium alloys.

4. A method of extruding a tube from an extrusion press having a movableram, an extrusion chamber, and a die which comprises:

a. providing 1. a dummy block having a socket concentric with the centerof the dummy block cross section, the socket being tapered outwardlyfrom front to rear;

2. a mandrel having a working section, a shaft section tapered inwardlyaway from the working section, and a raised shoulder portion of theworking section between the shaft and the nose of the mandrel, whereinthe average shaft cross sectional area is about equal to A SlP A where Ais the average cross sectional area of the working section,

P is the maximum fluostatic pressure in the container of the extrusionmachine, and

S is the frictional stress on the working section during extrusion;

b. positioning in the extrusion chamber a hollow metal billet whereinthe metal is selected from the group consisting of aluminum alloys andmagnesium alloys; c. positioning 1. the nose of the mandrel workingsection in the hole of the hollow metal billet in the extrusion chamberand 2. the shaft of the mandrel in the socket of the dummy block;

d. and continuously advancing the ram and mandrel, thereby extruding thebillet through the die to form a tube.

5. The method of claim 4 wherein the dummy block is attached to the ram.

6. A method of extruding a tube, formed of aluminum alloy or magnesiumalloy, from an extrusion press having a movable rarn carrying a dummyblock with an axially centered socket formed therein and an extrusioncontainer receiving the ram and dummy block, the extrusion containerbeing positioned against and aligned with a die plate having a dieorifice therein, which comprises:

a. providing a mandrel having a working section and a shaft section andplacing the shaft section in said socket in the dummy block, the shaftsection being frictionally held in said socket, the shaft crosssectional area being equal to A S/P A where A is the average crosssectional area of the working section,

P is the maximum fluostatic pressure in the container of the extrusionmachine, and

S is the frictional stress on the working section during extrusion,whereby the frictional stress on the working section during extrusiondoes not ordinarily exceed the .stress necessary to move the shaftsection out of said frictional engagement, the shaft section beingsmaller in diameter than the die orifice;

b. placing a hollow billet of aluminum alloy or magnesium alloy in theextrusion container,

c. advancing the ram and mandrel and positioning I the nose of themandrel working section in the hole of the hollow billet; and

d. continuously advancing the ram and mandrel throughout extrusion ofthe hollow billet through the die to form a tube.

1. A method of extruding tubes from an extrusion press having a movableram which comprises a. providing a mandrel which has a working sectionhaving a nose and a tail, and a shaft abutting the tail of the workingsection wherein the shaft cross sectional area is about equal toA2S/P1 + A2 where A2 is the average cross sectional area of the workingsection, P1 is the maximum fluostatic pressure in the container of theextrusion machine, and S is the frictional stress on the working sectionduring extrusion b. positioning the nose of the mandrel working sectionin hole of a hollow billet and the shaft in the socket of a dummy blockc. extruding the billet through a die to form a tube.
 2. The method ofclaim 1 wherein the extruded tubes are metal.
 2. a mandrel having aworking section, a shaft section tapered inwardly away from the workingsection, and a raised shoulder portion of the working section betweenthe shaft and the nose of the mandrel, wherein the average shaft crosssectional area is about equal to A2S/P1 + A2, where A2 is the averagecross sectional area of the working section, P1 is the maximumfluostatic pressure in the container of the extrusion machine, and S isthe frictional stress on the working section during extrusion; b.positioning in the extrusion chamber a hollow metal billet wherein themetal is selected from the group consisting of aluminum alloys andmagnesium alloys; c. positioning
 2. the shaft of the mandrel in thesocket of the dummy block; d. and continuously advancing the ram andmandrel, thereby extruding the billet through the die to form a tube. 3.The method of claim 2 wherein the metal is selected from the groupconsisting of aluminum alloys and magnesium alloys.
 4. A method ofextruding a tube from an extrusion press having a movable ram, anextrusion chamber, and a die which comprises: a. providing
 5. The methodof claim 4 wherein the dummy block is attached to the ram.
 6. A methodof extruding a tube, formed of aluminum alloy or magnesium alloy, froman extrusion press having a movable ram carrying a dummy block with anaxially centered socket formed therein and an extrusion containerreceiving the ram and dummy block, the extrusion container beingpositioned against and aligned with a die plate having a die orificetherein, which comprises: a. providing a mandrel having a workingsection and a shaft section and placing the shaft section in said socketin the dummy block, the shaft section being frictionally held in saidsocket, the shaft cross sectional area being equal to A2S/P1 + A2, whereA2 is the average cross sectional area of the working section, P1 is themaximum fluostatic pressure in the container of the extrusion machine,and S is the frictional stress on the working section during extrusion,whereby the frictional stress on the working section during extrusiondoes not ordinarily exceed the stress necessary to move the shaftsection out of said frictional engagement, the shaft section beingsmaller in diameter than the die orifice; b. placing a hollow billet ofaluminum alloy or magnesium alloy in the extrusion container, c.advancing the ram and mandrel and positioning the nose of the mandrelworking section in the hole of the hollow billet; and d. continuouslyadvancing the ram and mandrel throughout extrusion of the hollow billetthrough the die to form a tube.